Suggest — 3 marks
A student is designing a simple electric motor for a school project. The motor consists of a rectangular coil of wire placed between the poles of a permanent magnet. When a direct current is passed through the coil, it begins to rotate. The student wants to make the motor spin faster and more powerfully.
-
(a) Suggest one change the student could make to the motor to increase the force on the coil of wire.
[1 mark]
-
(b) Suggest how the student could make the motor spin faster, and explain your answer in terms of the motor effect.
[2 marks]
Show mark scheme
- (a) Increase the current through the coil / use a stronger power supply (1 mark)
- (a) Alternative acceptable answers: Use stronger magnets / increase the magnetic field strength / use a coil with more turns
- (b) Increase the current or use a stronger magnet / stronger magnetic field (1 mark)
- (b) This increases the force on the coil each time it rotates / the coil experiences a greater force due to the motor effect, causing it to accelerate and spin faster (1 mark)
Suggest — 5 marks
A student designs a simple electric motor using a rectangular coil of wire suspended between two magnetic poles. The coil is connected to a DC power supply via split-ring commutators and rotates continuously when switched on. The student observes that the motor rotates slowly and wants to increase its speed and power output.
-
(a) Suggest two modifications to the motor design that would increase the speed of rotation. Explain the physics principle behind each modification.
[3 marks]
-
(b) The student replaces the single rectangular coil with a coil containing four loops of wire (instead of one). Suggest why this change would increase the power output of the motor.
[1 mark]
-
(c) The student observes that if the split-ring commutator becomes worn or damaged, the motor's rotation becomes jerky and irregular. Suggest why this occurs and explain how the commutator normally ensures smooth rotation.
[1 mark]
Show mark scheme
- (a) Increase the magnetic flux density (use stronger magnets or bring poles closer) - this increases the force on the current-carrying coil (F = BIL), causing greater torque and faster rotation
- (a) Increase the current through the coil - this increases the force on the current-carrying wire, resulting in greater torque and faster rotation
- (a) Increase the number of coils/loops - more coils experience the motor effect simultaneously, producing greater total force and torque
- (b) More loops mean more current-carrying conductors in the magnetic field experience the motor effect at the same time, so the total force is greater, producing more power
- (c) A worn commutator causes poor electrical contact, so current does not flow continuously or consistently through the coil. Normally, the commutator reverses the direction of current every half rotation to keep the force in the same direction, maintaining continuous rotation. Poor contact disrupts this, causing jerky motion.
Explain — 4 marks
A student is building a simple electric motor for a school project. The motor consists of a rectangular coil of wire connected to a power supply, with the coil positioned between the poles of a permanent magnet. When the power supply is switched on, the coil begins to rotate continuously.
-
(a) Explain why the coil rotates when the power supply is switched on.
[2 marks]
-
(b) The student wants to make the motor rotate faster. Explain how increasing the current flowing through the coil would help achieve this.
[1 mark]
-
(c) Explain what would happen to the direction of rotation of the coil if the poles of the magnet were reversed.
[1 mark]
Show mark scheme
- (a) Current-carrying wire in a magnetic field experiences a force (the motor effect) / Lorentz force acts on the wire
- (a) The forces on opposite sides of the coil act in opposite directions, causing the coil to rotate / forces are in opposite directions so the coil turns
- (b) Increasing current increases the size/magnitude of the force on the wire / larger force causes faster rotation
- (c) The direction of the force on each side of the coil would reverse, so the coil would rotate in the opposite direction / reversing the magnetic field reverses the direction of rotation
Show — 4 marks
Figure 1 shows a simple electric motor consisting of a rectangular coil of wire placed between the poles of a permanent magnet. The coil is free to rotate about a central horizontal axis. When current flows through the coil, the motor effect causes forces to act on sides AB and CD of the coil.
-
(a) State two factors that affect the magnitude of the force acting on side AB of the coil due to the motor effect.
[2 marks]
-
(b) The magnetic field acts from the North pole towards the South pole. Current flows into the page at side AB and out of the page at side CD. Show that the forces acting on sides AB and CD of the coil will cause it to rotate about the axis.
[2 marks]
Show mark scheme
- (a) magnitude of the current
- (a) strength of the magnetic field (accept: magnetic flux density)
- (a) length of the conductor in the field (accept: number of turns on the coil)
- (b) forces on AB and CD act in opposite directions (e.g., AB down, CD up)
- (b) these opposite forces produce a turning effect / couple / moment about the axis
State — 3 marks
A student is investigating how an electric cooling fan works. The fan contains a simple DC motor. When the motor is switched on, a current flows through a coil of wire that is placed between the poles of a permanent magnet. This causes the coil to spin.
-
(a) State what must happen for a force to act on the wire in the coil.
[1 mark]
-
(b) State two factors that affect the size of the force on the wire.
[2 marks]
Show mark scheme
- (a) current flows through wire/conductor (in magnetic field)
- (b) strength of magnetic field / magnetic flux density
- (b) size of current
- (b) length of wire/conductor in the field
GCSE Perfect